1. Field of the Invention
The present invention relates to a Light Emitting Display (LED), and a display panel and a pixel circuit thereof, and more particularly to an Organic Light Emitting Diode (OLED) display and pixel circuit thereof.
2. Description of the Related Art
In general, an OLED display, which is a kind of LED for emitting light by electrically exciting a fluorescent organic compound, displays images by driving N×M organic light emitting pixels using a voltage programming method or a current programming method. An organic light emitting pixel has a multi-layered structure including an anode layer, an organic thin film layer, and a cathode layer. The organic thin film also has a multi-layered structure including an EMitting Layer (EML), an Electron Transport Layer (ETL), and a Hole Transport Layer (HTL) in order to enhance light emission efficiency by balancing electrons and holes. The organic thin film further include a separate Electron Injecting Layer (EIL) and a separate Hole Injecting Layer (HIL).
Methods for driving the organic light emitting pixels are generally classified into a passive matrix method and an active matrix method using Thin Film Transistors (TFTs). In the passive matrix method, anodes are perpendicular to cathodes and lines are selected and driven, while in the active matrix method, TFTs are coupled to respective pixel electrodes and the TFTs are driven by voltages maintained by capacitors coupled to gates of the TFTs. The active matrix method is classified into a voltage programming method and a current programming method based on the form of a signal which programs a voltage into a capacitor and maintains the programmed voltage.
In an OLED display, one pixel is composed of a plurality of sub pixels having respective colors, so that a color can be represented in various ways by combining colors generated by the plurality of sub pixels. In general, one pixel is composed of a sub pixel representing Red (R), a sub pixel representing Green (G), and a sub color representing Blue (B), and various colors can be represented by combinations of the red, green and blue.
In order to drive these sub pixels, a driving transistor for driving an OLED element for each sub pixel, a switching transistor, and a capacitor are required. In addition to this, a data line for transmitting a data signal and a power line for transmitting an operating voltage are further required. Therefore, there arises an increase in the number of transistors, capacitors, and lines required to form one pixel. Difficulties are encountered in arranging them inside the pixel. In addition, there arises a problem in that an aperture ratio corresponding to a light emitting area of the pixel is reduced.